How does role=note fit into this?

[medeaoblongata]

For an onscreen annotation element which is not a comment, a suggestion or a mark, I would (I suppose) choose role="note". Would that be correct?

Example,

<button aria-details="experimental">Do it</button>
<div id="experimental" role="note">This feature is experimental</div>

How about mentioning this in the section "Changes/clarification to existing markup".

Or else... what is the semantic purpose of role="note"? ARIA spec says "parenthetic or ancillary" which is a close match for a doc-footnote in the Web Annotation Data Model.

See w3c/aria#1629

[medeaoblongata]

BTW we have a case where several elements need to be annotated by the same thing. I haven't seen anything in your spec which would forbid this, but it complicates the 'round trip' implementation.